President Barack Obama said Thursday that his administration was required by law to give national security briefings, including classified information that is not disclosed to the public, to major presidential nominees.

This includes Republican Donald Trump, whom Obama has called unfit to serve as president.

During the past year of political campaigning, Trump has become known as a public speaker who often ignores prepared texts and relies instead on spontaneous remarks that are sometimes seen as unusually frank or critical of other American public figures. That has prompted concern in some quarters about how Trump would treat information gained from national security briefings.

"If they want to be president," Obama, speaking of party nominees, told reporters at the Pentagon, "they've got to start acting like a president, and that means being able to receive these briefings and not spread them around."

The main reason for the nominees' briefings, Obama said, is to ensure that a president-elect, whether a Democrat or a Republican, does not step into the job unprepared.

'Rigged' election

Trump, meanwhile, has told supporters at campaign rallies that he is concerned the U.S. election in November will be "rigged" against him. Obama dismissed that claim Thursday as "ridiculous."